The present invention relates to a method of connecting the output pads on an integrated-circuit chip and to a multichip module produced by using this method.
The most common way of arranging the bonding pads of integrated circuits is on the periphery of the latter. This allows short connections to be made to the circuit-supporting substrate. The main exception occurs in certain memories: the output pads 10 may be located elsewhere than on the periphery, for example along a mid-axis of the silicon chip 12 (FIGS. 1 and 2).
FIG. 3 shows an arrangement of the pads 14 in two rows on either side of the short axis of the chip 16; a few optional pads 18, generally with axial-pad redundancy, may be arranged on the periphery (this is the case for chips which are to be mounted by the so-called "flip-chip" or "C4" method). These arrangements are provided for mounting chips in the upside-down position (flip-chips) or for the wire-bonding of a lead frame laid on the chip and forming the outputs of an overmoulded package.
Such an axial arrangement of the pads leads to the impossibility of direct wire-bonding using aluminium or gold wires, as used conventionally.
The reason for this is that the wiring standards generally require a maximum distance between the ends of each bonded wire, for example less than 125 times the wire diameter (military standards) or 100 times the diameter (space standards). It is obvious that, with large chips (10 to 20 mm along the long side and 6 to 9 mm along the short side), it is increasingly problematical to carry out the wire-bonding with normal wires (diameters of 25 to 38 .mu.m), in particular in the configuration of FIG. 1, where a fan-out of the wires is necessary. In fact, in the example in FIG. 4, it may be seen that the wires d,d' may be longer than 5 mm.
A second drawback of these axial arrangements resides in the fact that it is impossible to mount the chips one on top of the other according to the so-called "crossed pairs" methods (see EP-A-0,489 643). The reason for this is that the upper chips then cover the output pads of the lower chips. One way of getting round this problem could be provided by a tile arrangement of rerouted chips (see EP-A-0,614,190). The rerouting of the outputs is achieved by depositing thin layers of the polyimide/copper type on that face of the chip having the output pads, which depositions, apart from their relative complexity have the drawback of being able to be carried out only by the chip manufacturer and not by the assembler.
In addition to the flip-chip, lead frame and rerouting methods mentioned hereinabove, it is also known, in order to connect the axial output pads on a memory chip, to adhesively bond the chip in the upside-down position on a ceramic substrate having an opening opposite the output pads on the chip. This latter solution allows several chip/substrate pairs to be stacked up in a superposed mounting, each substrate being equipped with lateral connections for connecting to a "squirrel cage" surrounding the assembly. The edges of the opening in the substrate opposite the output pads on the chip must be staged, by having a step to allow sufficiently compact stacking of the chip/substrate pairs. This opening is therefore difficult to produce. It involves, in particular, producing the substrate by means of co-fired layers. Furthermore, that portion of the lower face of the adjacent chip of the stack located opposite this opening is not in good thermal contact with a material promoting the removal of heat.
The object of the invention is to provide a novel technique for connecting the output pads on an integrated-circuit chip which reduces the impact of at least some of the problems mentioned hereinabove.